PSCI: Publish CPU ON event

This allows other EL3 components to subscribe to CPU on events.

Update Firmware Design guide to list psci_cpu_on_finish as an available
event.

Change-Id: Ida774afe0f9cdce4021933fcc33a9527ba7aaae2
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>
diff --git a/include/lib/el3_runtime/pubsub_events.h b/include/lib/el3_runtime/pubsub_events.h
index 8ef1a11..62550f8 100644
--- a/include/lib/el3_runtime/pubsub_events.h
+++ b/include/lib/el3_runtime/pubsub_events.h
@@ -10,3 +10,9 @@
  * This file defines a list of pubsub events, declared using
  * REGISTER_PUBSUB_EVENT() macro.
  */
+
+/*
+ * Event published after a CPU has been powered up and finished its
+ * initialization.
+ */
+REGISTER_PUBSUB_EVENT(psci_cpu_on_finish);